Wireless communication systems are changing the way people work, entertain, and communicate. For example, portable phones and other mobile devices have enabled highly mobile individuals to easily communicate. Such devices can transmit and receive both voice and data signals. As more features are added to these mobile wireless devices, users are able to receive a wider variety of information. This enhances the user's entertainment and more efficiently solves the user's business problems.
Data, such as computer files, graphics, video, and music may be sent from a remote location and received by mobile wireless devices located throughout a large (or “wide”) area. Such wide area uses generally require a series of fixed transceivers arranged to communicate with the mobile wireless devices. The wireless device is able to communicate only as long as it remains in contact with at least one of the transceivers.
While the use of such wide area systems is expanding, the use of local wireless communication systems is also growing. A local wireless communication system, for example, may configure the wireless devices in a single building, such as a residence, to share information. Such local wireless communication systems may enable computers to control peripherals without physical connections, stereo components to communicate, and almost any appliance to send and receive information to the Internet.
The amount of data being sent on both wide area and local communication systems is mushrooming, and it may quickly exceed the bandwidth available in the traditional communication bands. A relatively new communication technology (termed “ultra-wideband” technology) may provide assistance in meeting the ever-increasing bandwidth demands. An example of ultra-wideband technology is the communication system using an impulse radio system that is disclosed in U.S. Pat. No. 6,031,862, entitled “Ultra-Wideband Communication System and Method”. Impulse radio uses individually pulsed monocycles emitted at fractions of nanosecond intervals to transmit a digital signal. For many applications, the pulses are transmitted at extremely low power density levels, for example, at less than −30 dB. The generated pulses are so small that they typically exist in the noise floor of other more traditional communication systems.
Ultra-Wide band communication systems enable communication at a very high data rate, such as 100 megabits per second or greater, when operated in a small local area. Ultra-Wideband systems, however, must operate at extremely low power, typically transmitting signals at the noise level. These systems must operate at low power because they need to avoid interfering with the more established communication frequencies. The low power requirement restricts the size of each ultra-wideband cell. Thus, ultra-wideband cells generally are smaller than the cells in the more traditional continuous wave or carrier based systems.
The relatively small size of a cell in an ultra-wideband communication system necessitates a relatively dense placement of base station antennas. This high density of antennas may, under some circumstances, lead to cross-talk between the channels assigned to different users. This is especially true if the users are highly mobile. In this case, they will often travel across cell boundaries where the signals of two or more base stations overlap. Since this event will be relatively frequent with such small cells, user channels must be geographically separated to minimize the occurrence of channel interference. For example, if a particular channel is used in a cell, that channel should not be used in any other cell within several miles. Accordingly, since only relatively few of the communication channels can be allocated to each cell, the reuse distance determines the total capacity of the overall cell communication system.
The utilized bandwidth in conventional cells varies as a function of user demand. Since user demand can vary greatly from one time period to another, there are likely to be times when a particular cell is greatly under-utilized. There are also likely to be other times when that same cell is saturated, thereby causing undesirable drops in transmissions, connection refusals, and quality degradation. When a cell's bandwidth utilization exceeds system quality standards in a conventional communication system, the system operator typically will add another cell in the area to move some of the user traffic from the over-utilized cell to the new cell. Adding cells and antennas, however, can be a costly and time-consuming process.
Although ultra-wideband technology has the ability to decrease the impact of multipath interference, it is still subject to attenuation of the received signal as the signal passes between transmitter and receiver. For a point RF source, received signal strength varies as the inverse of the squared distance for open line of sight communications. In cluttered and mobile environments, the attenuation is more closely proportional to the inverse of the fourth power of the distance. This is due to multipath cancellation, which is present even in ultra-wideband signals. In either scenario, the attenuation of the signal can decrease the signal level to a value that is unsuitable for reliable data transfer.
Due in part to the deficiencies described above, conventional ultra-wideband communication systems risk poor quality of service, especially as a mobile unit moves from one location to another. Such systems also do not enable entirely efficient utilization of bandwidth and system resources.